The present invention relates to a system for providing pressurized fluid to a plurality of load circuits, and more particularly, to such a system in which one of the load circuits must be given "priority", i.e., the full demand for fluid by that circuit must be satisfied, with any excess flow available going to an auxiliary circuit.
Although the system of the present invention is equally adapted to any arrangement in which pressurized fluid is provided to a priority load circuit and an auxiliary load circuit, its use is especially advantageous in arrangements in which the priority load circuit comprises a vehicle hydrostatic power steering system, and the invention will be described in connection therewith.
It is known in the prior art to provide pressurized fluid to both a priority load circuit and an auxiliary load circuit by means of a single fluid source and a single priority flow control valve. See U.S. Pat. No. 3,455,210, assigned to the assignee of the present invention. It is also known in the prior art to provide pressurized fluid to both a priority load circuit and an auxiliary load circuit by means of a pair of fluid sources and some type of valving arrangement which typically directs all of the flow from the primary source to the priority load circuit, while directing fluid from the secondary source to the priority load circuit, only as needed. There is growing interest in systems of this type because of the trend toward requiring full fluid-linked hydrostatic steering systems to have some form of emergency steering capability, for example, in the event the primary fluid source fails.
One of the problems associated with dual pump systems of the type referred to previously is that the valving arrangement needed to "phase in" the flow of fluid from the secondary source to the priority circuit typically is specially designed for the particular application, and is fairly complex. Therefore, because the quantity of any one design of such a valving arrangement is generally small, the manufacturing cost per unit is quite high. In addition, such specially designed valving arrangements provide little flexibility in overall system design.